ProblemI got this error when trying to do a fresh install of Ubuntu 12.04 from USB. I get past the screen that asks if you want to download updates and install third party software, then I get stuck on the "Installation Type" screen.
The installer only sees /dev/sdb which is my USB and shows nothing in the partitions table. All the buttons are disabled. Clicking continue gives me the error "No root file system is defined".
Instead of installing, I went to "Try Ubuntu". This loaded up the Ubuntu desktop. Checking gparted and fdisk, I was able to find /dev/sda. Great! Unfortunately, trying to install gives me the exact same problem.
SolutionWhen you "Try Ubuntu", open up a terminal and type
sudo apt-get remove dmraid -yIt gave me some errors, but I ignored them. I ran the installer from the desktop and voila! The installer sees /dev/sda and I could install Ubuntu 12.04